Methods for the production of two-colored plastic molded skins by the “masking technique” are known.
EP 0 972 625 A1 is incorporated herein by reference and describes an apparatus for this purpose, namely, a forming tool for the production of two-colored plastic molded skins for parts of a motor vehicle, that is, especially dashboards, door trim, et cetera. The production method used here for producing sintered foils/slush skins is a rotational sintering, in which a plastic powder is in the rotating forming tool and melts onto correspondingly heated wall parts of the forming tool.
The apparatus shown here has two different lower parts of the forming tool boxes. The lower parts of the forming tool boxes, which in each case also comprise the powder bed, are detachably arranged with respect to the single upper forming tool box onto which the plastic powder melts. For the production of two-colored plastic molded skins, the first lower forming tool box has a shell-like projection, which projects into the interior space and with which—as with a mask—parts of the upper forming tool box are covered.
In a first step, a powder of a first color is melted on by rotational sintering with the regions of the upper forming tool box that lie under the shell-like covering not being coated and therefore remaining free from powder.
Before a second step, the first lower forming tool box is replaced by a second lower forming tool box, which contains a powder of a different color and does not have any coverings. In the subsequently repeated rotational sintering, the powder with the new color melts onto the region of the upper forming tool box that was previously kept clear and on the rear side of the already existing molded skin. After cooling and removal of the molded skin from the forming tool, a two-colored plastic molded skin is obtained on the side that faces the forming tool and is intended to become the outer side.
During the first working step, the shell-like covering of the lower forming tool box that projects into the interior space must for this purpose be carefully sealed off with respect to the upper forming tool box so that no sintering powder of the first color gets into the covering region. For this purpose, the shell-like projection has at its edges sealing elements which are formed so as to be elastic and/or hollow on the inside and can, if required, be charged with pressure or elastic media on the inside.
The great amount of work involved in changing the lower forming tool boxes and the difficult sealing of the mask, that is, the sealing between the edges of the shell-like covering of the lower forming tool box that projects into the interior space and the inner side of the upper forming tool box, are disadvantageous here. Thermal expansions and different pressure conditions of the seals, caused by complicated inner shapes of the upper forming tool box, lead to leakages and irregular abutment, which can make the color boundaries become blurred.
A similar situation exists in the case of the powder sintering method disclosed in United States patent publication 2006/0208389 with multiple powder application steps. In this method, the use of a mask and an inflatable seal are essential for the invention. The seal is located on the mask and lies laterally against and between separating webs and automatically clamps the mask when inflated. First, with the mask clamped in place, a first powder application step is carried out, then the mask is removed and powder sintering is carried out once more. Although the powder boxes required for this purpose can themselves be of a simpler design, the handling of multiple heavy boxes with different types of powder is in fact to be regarded rather as a disadvantage in mass production. United States patent publication 2006/0208389 is incorporated herein by reference.
U.S. Pat. No. 5,234,653 is incorporated herein by reference and describes a method for the production of multicolored plastic molded skins by sintering in which a plastic powder is in a forming tool divided by separating walls and melts onto correspondingly heated wall parts of the forming tool. The fixation of the separating walls takes place by means of pressure systems within the separating walls. By means of the pressure system, the separating walls are clamped with the aid of sealing gaskets between the lower forming tool and the upper forming tool and in this way provide a reliable sealing in the respective sintering operations with different color powders.
Here too there is a disadvantage because a relatively elaborate fixing of the separating walls has to be performed and, in addition, a high pressure force is required for the pressure systems.
U.S. Pat. No. 6,071,619 is incorporated herein by reference and discloses a method and an apparatus for carrying out a spray molding process for a two-color method in which a mask is placed on top of a web or upstanding edge which delimits certain regions of a spray mold and separates them from the other regions of the surface. The edges of the mask project slightly towards the spray side. The elastomeric spray material is then sprayed onto one side of the upstanding edge. Thereafter, the mask is removed and another elastomeric material, for example, one of a different color, is sprayed on. On the basis of the materials described in the exemplary embodiments, a method for spraying liquid components is disclosed. However, because of the required amounts of solvent alone, spraying methods with multiple different liquid materials are not easy to manipulate in industrial mass production processes.